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Long story short, we have some 2-wire BX running meandering circuits in our house to the lights and a few outlets (like 3 in the whole house) with no rhyme or reason. Should I make it a priority to replace the BX?

We replaced it in the 2 rooms we gutted, and when giving a room outlets I run a new cable and even try to move existing outlets from the BX to that new run, but should I try to rip it out other places like the lights? Or is it fine as is?

(Note: it is a post and beam house, crumbly horse hair plaster, no attic, and the base board is pinned down by two thick layers of wood flooring and gargantuan cut nails. Almost every project is a mini nightmare.)

I thought about putting a few of the lights on the outlet circuit, but I recall hearing that is bad form. Yet running a second circuit for just 1 or 2 light bulbs seems wasteful.

Long story short, we have some 2-wire BX running meandering circuits in our house to the lights and a few outlets (like 3 in the whole house) with no rhyme or reason. Should I make it a priority to replace the BX?

It depends. How is the insulation? If it's brittle, you should make replacement a priority. Also, if you need to connect grounded devices, those circuits should be replaced. Any circuits to bathrooms or kitchens should be replaced and protected with GFIs. For those circuits that you aren't replacing yet, you could install AFI breakers, which may provide additional protection against fires started by arcing. Since bedroom circuits are now required to have AFI, it won't be wasted money. Probably other circuits, too, but I'm not fluent in the code.

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I thought about putting a few of the lights on the outlet circuit, but I recall hearing that is bad form. Yet running a second circuit for just 1 or 2 light bulbs seems wasteful.

Bad form, but not prohibited. If you can wire them so they are on a different circuit from the receptacles in the same room, you can achieve the goal of not being totally without a light source in the room when a breaker trips. Make sure that the wiring to the light is rated for the ampacity of the circuit. If the breaker is 20A, you'll need 12AWG wire, even though the lamp will never draw more than a few amps.

I know you said it is 2-wire, but is the grounding intact from the armor? If yes, and the wire isn't brittle as mentioned by Fencepost, don't make it too high a priority, do it when it is convenient as that wire is likely still safe.

I know you said it is 2-wire, but is the grounding intact from the armor? If yes, and the wire isn't brittle as mentioned by Fencepost, don't make it too high a priority, do it when it is convenient as that wire is likely still safe.

AC -- ("Armored Cable") -- Replacement for "BX". Paper liner/wrap; has a bonding strip; jacket may be used as grounding conductor

MC -- ("Metal Clad") -- plastic liner; no bonding strip; insulated grounding conductor; jacket may NOT be used as a grounding conductor

Actual "BX" contains no ground conductor besides the jacket itself. If the jacket rusts where it is clamped to a box, that can represent a high-resistance connection. Therefore, the jacket shouldn't be trusted as a grounding conductor. AC, the replacement for "BX", contains a bonding strip between the jacket and the liner; that is intended to allow the jacket to be used as a grounding conductor.

Note that "BX" isn't an actual type defined by the NEC, but rather is a manufacturer's product designation. The only major types defined by the NEC are AC and MC. (I think. "BX" might have reference in the NEC, but without a valid grounding conductor it wouldn't be permitted for new install.)

There are other differences between AC and MC, including where they may be used and how they are to be installed. But that's too much minutiae for discussion here.

fencepost, your description of BX cable is completely different than what is called BX around here, and that is still avalible.

Is it actually labeled "BX" by the manufacturer? Or is that just what "everybody" calls it? Just because everybody calls it that doesn't mean that's what it is. My understanding is that it was originally a trademark, but has become genericized through popular use. "BX" is not a recognized type code by the NEC. AC and MC are recognized type codes.

From your article "Basic armored cable was developed in the early 1900s by Edwin Greenfield and Gus Johnson, who called their product BX cable. It has become a generic term for all armored cable." There is no mention in the OP that suggessts it is wiring from the early 1900's.

We gutted the kitchen and master bath so those rooms will be completely up to code. The other bathroom looks like it was remodeled in the 80s or later so electrically anyway it seems to be up to code and has a GFCI outlet (though the pipes are not pitched right and the town has no record of permits being pulled between 1865 and 2013...)

The "BX" cable is 2 wire, and I don't see a paper liner or liner of any sort. The wires are white and black, with the black neutral and the white hot, at least in the ones I tested. The insulation seems to be a woven cloth over rubber, and is so old and dirty it's hard to tell white from black. It is crumbly at the ends but other than that seems to be in good shape. The connections are twisted and then coated in tar or some nasty black crap.

Indoors it does not appear to have any rust. I used the shield as a ground for a few outlets just to get some 3-prongs and it seems to be working, my little tester was happy anyway. (I think only one of those is being used anymore but it's being used for my computer...) However, they also ran a bx line underground to the garage and it has rusted pretty much through. I think it is still powered, but I decided not to screw with it until I'm sure it is disconnected from the panel.

The whole house is a hodgepodge. The master bedroom had 2 outlets on 2 circuits, one 3-prong and one 2-prong. Most rooms had either one 2-prong or no outlets at all. I was lazy and just turned off the hall light to work on it and got a shock on the neutral line, which was apparently tied to the master bedroom light which was on. Is that bad? Oh, and I think that one circuit went from the dining room outlet to living outlet the upstairs to a master outlet then to living light to master light to hall light to ??? So if we used the microwave and a heater or AC upstairs we would trip a breaker.

Crumbly insulation at any point means replace the wire. You can't see most of it and if what you can see is degraded there's a good chance it can be degraded too.

BX shield can form a ground, but is no longer permitted to be used that way because at best it is not a good ground connection. If the runs and bends allow it, you might be able to pull new conductors (of the correct type) through the shield instead of opening the walls; just include a ground wire too and you should be good to go. Normal wire-pulling techniques are not easy doing this in such a small diameter space and you need to ensure that the insulation isn't being damaged by sharp edges on the way in- have a helper watch and feed the new wires as you pull and install grommets where needed.